Xylan is the major component of hemicellulose in plant cell wall. Xylan contains the D-xylose molecules at 1.fwdarw.4.beta. linkage as backbone, as well as side chains consisting of arabinofuranose, methylglucose and acetyl at 1.fwdarw.3 or 1.fwdarw.2 linkage. There is always a great amount of xylan generated during paper manufacture. Xylan was always discharged to streams or rivers due to lack of efficient recyclization. This leads to both the loss of resource and pollution of the environment. Moreover, xylan is also abundant in feeds of livestock, whereas it is not degradable in animal digestive tract for lack of proper enzymes. Therefore, xylan degradation would merely reduce pollution through good utilization of wastes resulted from paper production, but also enhance absorption of nutrients and growth of livestock with an economic advantage owing to a decrease of required feed amount.
In view of the above circumstances, it is believed that the development of applications of microbial xylanases is the best solution. It is disclosed in such as U.S. Pat. Nos. 5,116,746, 5,179,021, 5,407,827, 5,437,992, 5,498,534 and 5,591,304 that the addition of xylanase to pulps provided an improvement in bleaching effect and degradation of residual xylan. Further, it is disclosed in such as U.S. Pat. Nos. 5,429,828, 5,445,957, 5,612,055 and 5,662,738 that the addition of xylanase to hemicellulose-rich feeds for xylan degradation provides an improvement in the absorptive efficiency of nutrients and promoted growth of animals.
Xylanase (endo-1,4-xylanase EC 3.2.1.8) is capable of degrading xylan into oligosaccharide molecules or xylose molecules. Xylanase is universal in a variety of microbes which are capable of generating energy by degrading xylan in plant cell wall through xylanase and other polysaccharide-degrading enzymes. Also, xylanase was found present in insects and crustaceans feeding on plants. It is known that this enzyme is present in living organisms except for mammals (see A., Sunna and G., Antranikian, 1997, Crit. Rev. Biothch. 17(1):39-67; Peter Biely, 1985, Trends in Biotech. 3(11):286-290).
The microbes known as being capable of producing xylanase include Aspergillus niger, Aureobasidium pullulans, Cephalosporium, Bacillus stearothermophilus (cf. U.S. Pat. Nos. 5,591,619, 5,534,429 and 5,491,087). However, the common disadvantages, i.e., unsatisfactory activity, low production, high expense and complicated purification procedures, were found in those prior art references. Therefore, it is still necessary to develop new xylanase enzymes and a process for the production of the enzymes without the existing problems in the prior art.